The mean east-west velocity is a complicated function of latitude and height because of the transfer of
momentum both latitudinally and vertically by waves. “Wave-mean flow" interaction is a topic
too complicated to discuss here but see [1]}, [2]}, [3]}. Furthermore, the mean flow also contains the Hadley circulation. This is independent of time and therefore part of the mean, but it is a convective circulation, responding to spatially-varying heat sources, which is dynamic. Without wave-mean
flow interaction and the Hadley cell, the mean is a state of “solid body rotation". It is useful to look at this because, metaphorically,
solid body rotation is the wet plaster wall on which the wave-mean interactions and Hadley flows are painted and absorbed, or if one prefers a different metaphor, the dance floor on which the dynamics dances.
Furthermore, an improved approximation of the static part of the atmosphere subtracts from the mean the wave-mean flow
effects, which are dynamic.
